Running multiple applications at the same time is too confusing. She doesn't understand/notice the taskbar. She always tries to close one thing before starting another.

Machines with multiple functions/modes/uses are a generational thing. Up until the late 70's hardly anyone ever encountered a profoundly multi-function device before. Multitasking devices didn't come into mainstream use until later. Think about it, isn't it weird to have something that can be one kind of appliance, then after activating one control, have it change into a different kind of thing altogether? That would be like a fruit that resembles a pear when you peel it one way but an orange when you peel it another.

One can buy an actual crowbar/plier/wirecutter/hammer/wrench/screwdriver. A lot of people probably find it easier to have 1 tool for each purpose and grab the right one.

Does anyone remember the 1970's? Just how well would a TV/typewriter/stereo/VCR/telephone/phonograph/answering machine have sold? I think most people's eyes would've glazed over. Well, the personal computer has functions that subsume all of those devices, and the eyes of a lot of adults who were alive back then do glaze over when they encounter one.

"Just how well would a TV/typewriter/stereo/VCR/telephone/phonograph/answering machine have sold?"

That just blew my mind. I've never thought of a computer as _being_ all of those devices. I've always held the mental model of a computer being a really dumb robot of sorts that would perform separate actions.

Like a computer could _do_ all of those things, much in the same way I can drive, cook, balance a checkbook, etc.

I think this is a really fundamental computer science thing people whose names are not "Alan Turing" have to be taught (ideally in gradeschool) but few are. In many fields, the more expert you are, the bigger the wall of tools you have. (48 wrenches!!!). But a computer can literally become any other computer. That's fundamentally awesome and, for many, hard to understand.

Knowing that a particular interface is almost completely accidental is often the key missing piece in going from being utterly helpless to solve a problem to being able to puzzle it out.

Knowing that a particular interface is almost completely accidental...

Yes! A lot of interfaces are largely arbitrary fictions! If more people understood this, there would also be fewer fan boyish arguments.

Also suggests a metric/strategy: minimize the amount of fiction in your UI and maximize the fundamental principles. (Example, sliders are well understood, because most people understand basic geometry.)

Yes, but you have this mental furnishing of "a dumb robot...that would perform" multiple functions. Earlier generations don't have that. Instead, they have this TV/typewriter/stereo/VCR/telephone/phonograph/answering machine that will also come to do god only knows what else in the near future.

Only a matter of time before some biotech genius invents a fruit that has different tastes depending on how you peel it, then it's my turn for a brain meltdown.